1. Field of the Invention
The present invention relates to the photochemical oxidation of water.
2. Discussion of Prior Art
As stocks of conventional fuels such as coal, oil and natural gas become more and more depleted, greater effort is being made to find practical ways of using sunlight which is incident on earth as a direct source of energy. Known methods of using sunlight as an energy source are, for example, the use of photo-electric cells in "solar batteries" to produce electrical energy from sunlight and the use of solar panels for heating purposes.
A disadvantage of these methods is that they are not capable of giving long term storage of energy.
The most useful method of solar energy utilisation would be a cyclic chemical process such as that used by plants in photosynthesis because such a method would be able to make the fullest possible use of sunlight which reaches the earth's surface only at intermittent intervals.
At present, other than the processes which occur in plants themselves, there are no reported methods in which visible light is used to oxidise water into oxygen and in which the electrons which are released by the water in such a process are captured by an electron acceptor to form a compound which acts as a store of energy and which may subsequently be used as a chemical fuel.
The photooxidation of water to oxygen is a very difficult process, and as far as we are aware there are only two systems which are known to work. These systems involve ultra-violet (.lambda.254 nm) photolysis of iron (III) and cerium (IV) salts in acidic solution (see V. Balzani and V. Carassiti, Photochemistry of Coordination Compounds, Academic Press, London (1970)).
It is not feasible to use a dye to sensitise these reactions to visible light because of the involvement of hydroxyl radicals, which react with the dye to form hydroxylated products. In order to avoid formation of radicals it is necessary to devise some kind of charge storage whereby the oxidant in the reaction system can be raised to a state in which it possesses more than one oxidation equivalent. The formation of one molecule of oxygen from water requires overall the transfer of four electrons.
Plants are known to use a manganese complex to perform a photooxidation of water to produce oxygen. M. Calvin in Science 1974, 184, P375 has suggested using a binuclear manganese (IV) compound such as di-.mu.-oxotetrakis (2, 2'-bipyridyl) manganese (IV, III) to simulate the photooxidation process. Irradiation of this compound or the related 1,10-phenanthroline complex with light of a wavelength of about 300 nm in acidic solution does not produce oxygen (see S. R. Cooper and M. Calvin, Science, 1974, 185, 376, and Y. Otsuji et al., Chem. Letts, 1977, 983). Furthermore, we have found that the corresponding manganese (II) complexes cannot be photooxidised to higher valence complexes (see R. G. Brown, A. Harriman, and G. Porter, J.C.S. Faraday Trans. II, 1977, 73, 113) so that with these complexes of manganese a cyclic system cannot be conceived.
Manganese porphyrin and phthalocyanine compounds are reviewed in a paper by L. J. Boucher, Coord. Chem. Revs, 1972, 7,289, and show intense absorption in the visible region. Thus, these compounds can be excited directly with light corresponding to the solar spectrum and some photoreduction of manganese (III) phthalocyanine (Mn .sup.III Pc) in organic solvents has been reported by G. Engelsma, et al., J. Phys. Chem, 1962, 66, 2517. However, no photooxidation of the manganese (III) complexes to manganese (IV) complexes takes place under such conditions and no oxygen is generated.
T. S. Glikman and L. N. Zavgorodnyaya, Biokhimiya, 1973, 38, 101, describe the use of quinones to assist oxidation of a manganese (II) pheophorbide or pheophytin complex to the corresponding manganese (III) pheophorbide or pheophytin complex in highly basic solution. However, in the systems described in this paper, the pheophorbide or pheophytin ligands are unstable to further oxidation and it would appear that these ligands would themselves be oxidised in the presence of an oxidant, rather than the central manganese (III) atom. As a result, in these reaction systems no manganese (IV) complexes are produced. Furthermore, in the presence of the hydroxide ions of the highly basic solution the manganese (III) complexes react rapidly with the hydroxide ions and are reduced back to complexes of manganese (II) which is the stable oxidation state of manganese under these conditions.
To our knowledge, neither the photochemical production of a Mn (IV) compound (of any description) nor the successful photogeneration of O.sub.2 using a Mn complex has been reported in the literature.